Th–U–total Pb geochronology of authigenic monazite in the Adelaide rift complex, South Australia, and implications for the age of the type Sturtian and Marinoan glacial deposits

The Adelaide rift complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemo-stratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses and Rodinia supercontinent reconstructions, but reliable depositional age constraints have been extremely limited. We report results of in situ Th–U–total Pb (electron microprobe) dating of detrital and authigenic monazite in two samples from the Umberatana Group (Sturtian Holowilena Ironstone and pre-Marinoan Enorama Shale) in the Central Flinders Ranges. Several texturally and chemically distinct detrital and authigenic populations are recognized. Detrital dates range from 1600 Ma to 760 Ma and most relate to well-known orogenic or igneous events in surrounding cratonic regions. Authigenic monazite grew in three or more pulses ranging from 680 Ma to 500 Ma. The date of 680 ± 23 Ma (2σ) for the earliest generation of authigenic monazite in sandstone from the Enorama Shale (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) supports proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China, which bracket the presumed Marinoan equivalents between 655 and 635 Ma. This age is inconsistent with a Re–Os isochron age of 643 ± 2.4 Ma (2σ) on shales near the bottom of the Sturtian–Marinoan interglacial succession, stratigraphically > 3000 m below the Enorama Shale sample, and militate against the hypothesis that the type Marinoan is correlative with the 580 Ma Gaskiers glaciation. Monazite growth near 600 Ma and again at about 500 Ma probably represent hydrothermal fluid-flow events, the latter of which also corresponds to the well-known Delamerian Orogeny during which the Adelaide sediments were folded into their present structural pattern

EXTRAPOLATION OF NUCLEAR WASTER GLASS AGING

Increased confidence is provided to the extrapolation of long-term waste form behavior by comparing the alteration of experimentally aged natural basaltic glass to the condition of the same glass as it has been geologically aged. The similarity between the laboratory and geologic alterations indicates that important aging variables have been identified and incorporated into the laboratory experiments. This provides credibility to the long-term predictions made for waste form borosilicate glasses using similar experimental procedures. In addition, these experiments have demonstrated that the aging processes for natural basaltic glass are relevant to the alteration of nuclear waste glasses, as both appear to react via similar processes. The alteration of a synthetic basaltic glass was measured in MCC-1 tests done at 90/sup 0/C, a SA/V of 0.1 cm/sup -1/ and time periods up to 182 days. Tests were also done using (1) MCC-2 procedures at 190/sup 0/C, a SA/V of 0.1 cm/sup -1/ and time periods up to 91 days and (2) hydration tests in saturated water vapor at 240/sup 0/C, a SA/V of approx. 10/sup 6/ cm/sup -1/, and time periods up to 63 days. These results are compared to alteration observed in natural basaltic glasses of great age. 6 references, 6 figures, 1 table

Determining the Pyroxene Mineralogies of Vestoids

Bulk pyroxene compositions were calculated for a number of V-type asteroid spectra using formulae derived by Burbine et al. These formulae were derived by analyzing HED (howardite, eucrite, and diogenite) meteorites and calculate bulk Fs (mol%) and Wo (mol%) contents using derived band centers. Using HEDs with known bulk pyroxene compositions, the uncertainty in the predicted Fs contents was determined to be ±3 mol%, and the uncertainty in the predicted Wo contents was ±2 mol%. V-type asteroids tend to have interpreted pyroxene mineralogies consistent primarily with eucrites and howardites. We investigate why diogenitic mineralogies appear so rare among ∼5–10 km V-type asteroids but are much more commonly present among HED meteorites. One possibility is that diogenitic intrusions are extremely “thin” but widespread in Vesta’s eucritic crust. In this scenario, Vestoids (V-type asteroids thought to be derived from Vesta) would be expected to be solid fragments of Vesta. Another possibility is that Vesta’s upper crust has been significantly shattered and diogenitic material would be much less common than the eucritic material in the crust. Vestoids would then be expected to be rubble piles. The belief that most asteroid families were shattered at least twice would argue that Vesta’s crust is also shattered and that Vestoids are rubble piles

A New Albite Microanalytical Reference Material from Piz Beverin for Na, Al and Si Determination, and the Potential for New K-Feldspar Reference Materials

Determination of alkali elements is important to Earth scientists, yet suitable and reliable microanalytical reference materials are lacking. This paper proposes a new albite reference material and evaluates the potential for future K-feldspar reference materials. The proposed Piz Beverin albite reference material from Switzerland yields a homogeneous composition at the centimetre- to micrometre-scale for Si, Al and Na with 2% relative local variations of Na, K and Ba contents. The three other investigated sets of K-feldspar crystals are yellow sanidine crystals from Itrongay (Madagascar). Despite distinct compositions, EPMA confirms they are each homogeneous at the centimetre to micrometre scale for Si, Al and K and have no apparent inclusions; further investigation to find larger amounts of these materials is therefore justified

A New Albite Microanalytical Reference Material from Piz Beverin for Na, Al and Si Determination, and the Potential for New K-Feldspar Reference Materials

Determination of alkali elements is important to Earth scientists, yet suitable and reliable microanalytical reference materials are lacking. This paper proposes a new albite reference material and evaluates the potential for future K-feldspar reference materials. The proposed Piz Beverin albite reference material from Switzerland yields a homogeneous composition at the centimetre- to micrometre-scale for Si, Al and Na with \u3c 2000 μg g-1 total trace elements (mostly heterogeneously distributed Ca, K and Sr). EPMA and LA-ICP-MS measurements confirm a composition of 99.5(2)% albite component, which is supported further by bulk XRF measurements. A round robin evaluation involving nine independent EPMA laboratories confirms its composition and homogeneity for Si, Al and Na. In addition, a set of five distinct clear K-feldspar samples was evaluated as possible reference materials. The first two crystals of adular and orthoclase yield unacceptable inhomogeneities with \u3e 2% relative local variations of Na, K and Ba contents. The three other investigated sets of K-feldspar crystals are yellow sanidine crystals from Itrongay (Madagascar). Despite distinct compositions, EPMA confirms they are each homogeneous at the centimetre to micrometre scale for Si, Al and K and have no apparent inclusions; further investigation to find larger amounts of these materials is therefore justified

Determining the Pyroxene Mineralogies of Vestoids

Bulk pyroxene compositions were calculated for a number of V-type asteroid spectra using formulae derived by Burbine et al. These formulae were derived by analyzing HED (howardite, eucrite, and diogenite) meteorites and calculate bulk Fs (mol%) and Wo (mol%) contents using derived band centers. Using HEDs with known bulk pyroxene compositions, the uncertainty in the predicted Fs contents was determined to be ±3 mol%, and the uncertainty in the predicted Wo contents was ±2 mol%. V-type asteroids tend to have interpreted pyroxene mineralogies consistent primarily with eucrites and howardites. We investigate why diogenitic mineralogies appear so rare among ∼5–10 km V-type asteroids but are much more commonly present among HED meteorites. One possibility is that diogenitic intrusions are extremely “thin” but widespread in Vesta’s eucritic crust. In this scenario, Vestoids (V-type asteroids thought to be derived from Vesta) would be expected to be solid fragments of Vesta. Another possibility is that Vesta’s upper crust has been significantly shattered and diogenitic material would be much less common than the eucritic material in the crust. Vestoids would then be expected to be rubble piles. The belief that most asteroid families were shattered at least twice would argue that Vesta’s crust is also shattered and that Vestoids are rubble piles

Basalt glass: an analogue for the evaluation of the long-term stability of nuclear waste form borosilicate glasses

The long-term stability of nuclear waste form borosilicate glasses can be evaluated by understanding the processes that effect the long-term alteration of glass and by comparing laboratory alteration of synthetic basalt and borosilicate glasses with the observed stability of naturally occurring basaltic glasses in diverse geologic environments. This paper presents detailed electron microprobe analyses of naturally altered basaltic glasses (with maximum ages of 10,000 to 20 million years) from low-temperature environments. These results are compared to laboratory data on the corrosion of a synthetic basaltic glass in MCC-1 tests (90/sup 0/C, a SA/V of 0.1 cm/sup -1/ and time periods up to 182 days), MCC-2 tests (190/sup 0/C, a SA/V of 0.1 cm/sup -1/ and time periods up to 210 days) and hydration tests in saturated water vapor (240/sup 0/C, an estimated SA/V of approx. 10/sup 6/ cm/sup -1/ and time periods up to 63 days). Additionally, laboratory-induced hydration alteration of synthetic basalt and borosilicate glasses is compared. These preliminary experiments provide evidence that the alteration processes observed for natural basalt glasses are relevant to understanding the alteration of nuclear waste glass, as both appear to react via similar processes. 12 references, 6 figures, 1 table